The present invention relates to a force sensor assembly, and more particularly relates to a force sensor assembly, which is able to provide accurate measurement of a force.
An air bag system is mounted on a vehicle so as to provide safety for a passenger in case of a collision. A sensor is installed in a side seat in order to control the air bag system. A force sensor is typically selected for this sensor, which is able to measure the weight of a passenger when he is seated on the seat. This force sensor, which detects a seated passenger, generates a signal for controlling inflation of the air bag system. The control includes a case where a system prevents an air bag from inflating if the system determines that a passenger is a child, and another case where a system adjusts speed of inflation of an air bag according to the weight of a passenger, for example.
Patent document 1 discloses a technique associated with a force sensor assembly. This technique employs an upper rail, on which a seat cushion frame is disposed, is slidably supported on a seat track. The seat frame and upper rail have respective through holes, which are aligned with each other. A nut is tightened onto a threaded portion of the force sensor, which is inserted through the through holes. When a passenger is seated on a seat, the seat cushion frame pivots relative to the upper rail, increasing a distance between the seat cushion frame and the upper rail. This produces a tensile force acting on the threaded portion. In this way, the force sensor detects the force. The technique described above, which requires a mechanism that allows the seat cushion frame to pivot relative to the upper rail, inevitably renders the assembly complex.
Accordingly, it may be preferable in terms of simplification to place another type of sensor, which senses a compressive force acting downward instead, between a seat cushion frame and an upper rail, as disclosed in patent document 2.    Patent document 1: 2000-203324 (paragraphs 0024, 0029, FIG. 3)    Patent document 2: U.S. Pat. No. 3,268,128 (08-005475) (paragraphs 0013 to 0017, FIG. 1)
However, because the force sensor assembly disclosed in the patent document 2 requires that tightening of a nut onto the threaded portion does not have an adverse effect on the force sensor, it will be necessary to introduce more complex operation for tightening the nut. If an excessive torque is imposed on the nut, for example, it will affect adversely the force sensor to provide less accuracy due to a tensile stress axially acting on the threaded portion. In addition, because an origin of the force sensor is shifted, an available range for detecting a force will be narrowed, which leads to difficulty in implementing highly accurate detection.
There is also another problem that decreases accuracy for detection. It may be that the excessive torque induces torsion about an axis of the force sensor.
On the other hand, when tightening is carried out paying attention to an effect on the force sensor, it may possibly occur that the torque falls short to create looseness between the seat cushion frame and the upper rail, which is a cause for incomplete fastening.
A force sensor assembly disclosed in the patent document 1, which is secured to a seat frame and a sliding frame, tends to suffer preload when it is mounted. There are several causes for this preload, such as an error in parallelism of the sliding frame, variation in dimensions for a sensor mounting area of the seat frame which is created during its fabrication and a displacement of mounting position of a seat onto a vehicle body. In this case, distortion caused by the preload in the force sensor may add up to a false detection including the distortion in addition to the true weight of a passenger, or may create a measurement error due to deterioration of accuracy of the force sensor.